1. Field of the Invention
This invention relates generally to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated block on the disk, and more particularly to such a disk drive with an improved clock for writing the data.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated blocks or islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional continuous media wherein a single “bit” may have multiple magnetic domains separated by domain walls. To produce the required magnetic isolation of the patterned blocks, the magnetic moment of the regions between the blocks must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the blocks. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., “Patterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording?”, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, pp. 990–995.
Disk drives typically use a rotary voice-coil-motor (VCM) actuator for positioning the read/write heads. A servo control system receives servo positioning information read by the read/write heads from the data tracks, typically from equally-angularly-spaced servo sectors that extend generally radially across the tracks. The servo control system generates control signals to the VCM to maintain the read/write heads on track and move them to the desired tracks for reading and writing of data. The servo sectors also contain servo timing marks (STMs) that indicate the start of the servo sectors. The STMs are often used as a reference clock signal for the write clock which controls the timing of the write pulses from the disk drive write head.
To assure that the disk drive write head magnetizes the blocks on the patterned media, the media must be patterned perfectly with a single accurate period, and the effective motor speed of the spindle supporting the disks must be highly stable so that the write clock is perfectly synchronized with the blocks as they pass beneath the write head. While the STMs are often used as the reference clock input to the write clock, U.S. Pat. No. 6,754,017 B2, assigned to the same assignee as this application, describes a magnetic recording disk drive with patterned media that uses a special pattern-sensor that senses the data blocks before they pass beneath the write head and generates a write-clock signal.
Disk drives experience rotational vibration and disturbance forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system. These disturbances affect the spindle motor speed, causing small speed variations or “jitter”, which in turn affects the timing of the reference clock input. The spindle motor jitter is especially problematic in patterned media drives because of the requirement of precise timing of the write pulses to the patterned data blocks.
What is needed is a magnetic recording disk drive with patterned media that has an accurate write-clock signal that is not significantly affected by rotational disturbances so that the data can be accurately written to the patterned data blocks.